Laminated resin moldings formed by laminating a polyamide-based resin layer and a fluorine-containing ethylenic polymer layer are regarded to be suited for use as multilayer molded articles required to have mechanical properties and low permeability against liquid agents capable of deteriorating polyamide-based resins, for example solvents and fuels, thus suited for use as automotive fuel piping tubes or hoses or tanks. In particular, multilayer molded articles comprising a polyamide-based resin layer as an outer layer and a fluorine-containing ethylenic polymer layer as an inner layer, if necessary together with a further fluororesin layer on the inner layer side are expected to serve as composite materials having such fluorine-containing ethylenic polymer characteristics as heat resistance, oil and chemical resistance and low permeability against solvents and fuels, as well as such polyamide-based resin characteristics as high strength, high toughness, light weight, good processability and, in particular, flexibility. The term “liquid agents” as used herein for short means “liquid agents capable of deteriorating polyamide-based resins, for example solvents and fuels”.
However, the polyamide-based resin layer and the layer comprising a fluororesin such as a fluorine-containing ethylenic polymer are generally low in interlayer adhesiveness. Therefore, attempts have so far been made to increase the interlayer adhesion strength. Thus, for example, Japanese Kokai Publication Hei-05-8353 discloses a multilayer tube comprising a polyamide resin as an outer layer and a fluororesin as an inner layer. In the gazette cited above, it is proposed that, for securing a certain level of adhesion strength between the polyamide resin layer and the fluororesin layer, crosslinking structures are introduced among molecules in both layers by irradiation. However, this technology cannot be applied to the coextrusion process, which is the usual technique of producing multilayer tubes, hoses or the like.
A technology has also been developed according to which the resin layer to be adhered to a polyamide-based resin is made of a blend with a fluororesin. Thus, for example, Japanese Kokai Publication Hei-07-53823 discloses a multilayer tube comprising a polyamide resin as an outer layer and a fluororesin as an inner layer, wherein a resin composition comprising both a specific polyamide resin and a fluorine-containing resin is laminated to the polyamide resin layer so that it may serve as an adhesive layer with the inner layer. When this method is used, however, the morphology of the adhesive layer changes according to the molding conditions due to the intrinsically poor compatibility between the constituent resins of the adhesive layer, namely the polyamide resin and fluorine-containing resin, and the morphology change influences on the cohesive force of the adhesive layer itself and on the adhesion strength thereof with the other layers. Accordingly, such problems arise as a tendency toward variation in adhesion strength according to environmental factors, such as molding conditions and temperature conditions during use, and difficulty in securing a constant quality level. In addition, this is not a technology to increase the adhesiveness of the polyamide-based resin and the fluororesin itself but one merely utilizing the adhesiveness of the polymer blend. The use of such polymer blend in lieu of the fluororesin results in an impairment in those excellent characteristics of the fluororesin.
To solve these problems, attempts have been made to improve fluororesin species themselves, and various fluororesin materials have been proposed. Thus, for example, the WO 99/45044 pamphlet discloses a laminate comprising a fluorine-containing ethylenic polymer having carbonate groups and/or carbonyl halide groups as a fluororesin to be laminated with the polyamide resin.
However, even when the initial interlayer adhesion strength between the polyamide-based resin and fluorine-containing ethylenic polymer is improved by these technologies, the decrease in adhesion strength with time, in particular the decrease in interlayer adhesion strength of the molded articles upon soaking with liquid agents capable of deteriorating polyamide-based resins, for example solvents and fuels, cannot be avoided. For example, it is a typical problem that the interlayer adhesion strength decreases in multilayer tubes in automotive fuel piping systems and, accordingly, the fuel permeability of the tubes increases with time. In this case, the cause is presumably that, in the condition of use in which multilayer tubes are in continuous soak with the automotive fuel, lower alcohols and like components contained in gasoline gradually permeate the fluororesin layer and, as a result, deteriorate the polyamide-based resin. Therefore, it is a matter of concern that the multilayer molded articles for use in transporting and storing liquid agents which are capable of deteriorating polyamide-based resins, for example solvents and fuels, should retain their interlayer adhesion strength. Further, it is necessary to effectively suppress the permeability in multilayer tubes and like moldings, in particular, to such solvents and fuels containing lower alcohols and like components.
Furthermore, even when such an improved fluorine-containing ethylenic polymer is used in combination with a commercially available polyamide-based resin, the mutual compatibility of the resins is insufficient, hence the cold temperature impact resistance is poor; thus, the use of a polyamide-based resin with a high plasticizer content is considered to raise a problem. Therefore, in producing multilayer tubes for automotive fuel piping, for instance, using such a combination, a polyamide-based resin with a high plasticizer content cannot be used as an outer layer so that the tubes must become hard; as a result, troubles arise in mounting the tubes on automobiles and the like. If a plasticizer-free polyamide-based resin is used, the resin pressure has to be increased, so that a problem arises that the molding speed (tube line speed) cannot be increased.
In view of the foregoing, there is a demand for laminated resin moldings which can impart to molded articles with good flexibility and mechanical properties as well as a high level of durability against heat, various chemical substances and other external environmental factors by providing a polyamide-based resin as an outer layer and which can markedly reduce in permeability against liquid agents capable of deteriorating polyamide-based resins, for example solvents and fuels, by providing a fluorine-containing resin layer as an innermost layer, in particular laminated resin moldings which can be molded by coextrusion molding and can produce such good multilayer molded articles in which the interlayer adhesion strength can be maintained over a prolonged period of time.